To decrease an amount 70% what single multiplier would you use

Tom wants to order tickets online so that he and three of his friends can go to a water park the cost of the tickets is 16.00 per person there is also a 2.50 one-time service fee for ordering tickets online write an expression in term of n that represents the cost for n ordering tickets online

What is the sum of the first 8 terms of the geometric series

Need plz been trying to do it for a long time and still haven’t succeeded